Research Of Organic Dyes Photocatalytic Degradation By Bismuth Tungsten

Environmental pollution has been the most important problem with the rapiddevelopment of industry recent years. In particular, the rapid development of thedye industry and dyeing industry also increasing the variety and quantity of dyesdischarged into water bodies. Organic dyes are important class of industrial dyes.Because of its high chromaticity, strong toxity and difficulty degrade, and it canbring a serious threat to our bodies. People began to study the treatment methodsof the dye wastewater. Traditional techniques such as adsorption, biologicaloxidation generally have poor results to remove dyes from dye wastewater inorder not to degrade completely. In recent years，a large number of investigationshave focused on photocatalytic oxidation technology since researchersdiscovered the photocatalytic splitting of water on the TiO₂electrodes. Bi₂WO₆has been recognized as a photocatalyst for the degradation of organic dyes undervisible light irradiation among photocatalyst which are researched recently.In this paper, bismuth nitrate （Bi（NO₃）₃·5H₂O） and sodium tungstate（Na₂WO₄·2H₂O） were used as raw materials, hollow structure Bi₂WO₆nanospheres which have photocatalytic properties were successfully preparedwith the support of the citric acid and NaHCO₃. Photocatalytic degradation ofrhodamine B solution with different concentrations and methyl orange （MO）solution with different pH value under visible light irradiation （500W xenonlamp） with Bi₂WO₆nanospheres as photocatalysts. By studying the degradationprocess and degradation intermediates, we can analysis the degradationmechanism of the two different kinds of dyes. In this experiment, X-raydiffraction （XRD）, scanning electron microscopy （SEM）, Transmission ElectronMicroscopy （TEM） and UV-Vis diffuse reflectance spectroscopy （UV-Vis） were used to analysis the structural characteristics, morphology and optical propertiesof the Bi₂WO₆nanospheres. In addition, UV-Vis diffuse reflectancespectroscopy （UV-Vis）, high performance liquid chromatography （HPLC） andchromatography mass spectrometry （LC-MS） were used to analysis thedegradation process and degradation intermediates of organic dyes and proposethe degradation mechanism finally.Experimental and analytical results are shown as followings:（1） Bi₂WO₆nanospheres, which are prepared by hydrothermal method, areorthorhombic, good crystalline, high purity and the morphology is relativelyuniform. Scanning electron micrographs and transmission electron microscopyshows that the hollow structure microspheres are deposited by a large number ofnanosheets with the length of about150²00nm. The selected area electrondiffraction （SAED） shows these microspheres are single crystal structure.（2） The RhB solution in different concentrations（5mg/L,10mg/L,20mg/L）are photocatalytic degradation by Bi₂WO₆nanopowders which are prepared inthe previous step. The results show that RhB solution can be photodegradedgradually under visible light and the greater the concentration, the longer thedegradation time. Photodegradation process is analysised by three differentspectrums: Photocatalytic degradation process of the RhB solution is the resultof the role of the de-ethylation and hydroxylation process, and the concentrationof the solution have a direct impact on the two processes. The photocatalyticdegradation process of the RhB solution is very complex: firstly, groups outsidethe benzene ring are detached, and then the benzene ring is gradually cracked tomake the RhB solution degradated a large number of small moleculesultimately.（3） Methyl orange （MO） solution in different pH values （pH=5,3,1） arephotocatalytic degraded by Bi₂WO₆nanopowder. Analysis results show that thesmaller the pH of the solution, the faster the degradation rate. MO is a typicalazo dye and the azo bond（－N=N－）is its chromophore. Its azo bond isdestroyed under acid condition, so its structure changed from azo structure toquinoid structure. In addition, acid can affect the structure of the photocatalystand the activity of hydroxyl （·OH）, so it can also affect the photodegradationefficiency of MO. Comprehensive analysis results show that the photodegradation of methyl orange mainly is the process of the benzene ring isattacked by the hydroxy and finally make it degradate many colorless, non-toxic,harmless small molecules.